This invention relates to a filament bypass circuit and more particularly to a filament bypass circuit employing a voltage operated switch to maintain current flow when the filament fails.
In strip lighting configurations a plurality of lights are strung in series, for example, as in Christmas tree and other ornamental applications. When one bulb fails, the whole string of lamps goes dark. To replace the defective bulb it is necessary to inspect and check each bulb. In many of these arrangements, the bulbs are inexpensive and the inconvenience of locating the failed bulb may be acceptable. In certain other situations, this inconvenience may be unacceptable, with the result that a variety of arrangements for identifying the defective bulb are in existence, and in some configurations there is provision for maintaining the remaining bulbs lit until the defective bulb can be changed.
In some cases, it is necessary or desirable to use special light bulbs having characteristics which are required in particular applications, and an example of such bulbs is the halogen lamp which due to the presence of a halogen gas in the envelope permits higher intensity current flow through the filament with the result that much brighter light is obtainable in a very small bulb size as well as producting a color rendition which is highly desirable in certain applications. Examples of such specialized applications would be lighting of works of art in museums, lighting in discotheques, and stage lighting. The halogen lamp may be used alone or in strings. The halogen lamp requires a low voltage, such as 12 volts, for its operation, and when used alone or in parallel in order to avoid the problem of locating the failed bulb, a bulky and heavy transformer is required to reduce the voltage.
When the halogen lamp is employed in strings, the reduced voltage is obtainable by connecting them in series so that the drop across each lamp is at the rated value. However, in the case of the failure of a single lamp, the whole string goes dark without indicating which lamp has failed. When a bypass circuit, of design heretofore available, is employed to maintain the other lamps lit after a failure has occurred it may not even be noticeable that one lamp is dark, since there is no indication of such a failure other than one lamp being dark. In the environments where halogen lamps are or may be employed, it may not be immediately noticeable that a failure has occurred. In addition, in many of the bypass circuits known up to now, continuation of current flow results in an increased voltage across the remaining lamps with the result that the life of those lamps may be shortened by a very substantial amount. In the high intensity halogen lamp, small increases over rated voltage could cause a considerable shortening of the lives of the remaining lamps. Since the halogen as well as other specialized lamps may be quite expensive, present arrangements for maintaining current flow are not completely satisfactory.